In rare cases the receiving email server of your user marks our mail as spam. Sometimes the mail appears in the spam folder. Sometimes they are rejected and not shown. This lies within the responsibility of the mail hoster and your adress-quality.
To make sure your adress-quality remains high and your mails don't get blacklisted make sure to process your bounces (if a mail-adress is not existant - you will often receive a bounce mail to your contact which explais what went wrong)
So - too keep your SPAM-Rate low and also guarantee successful delivery to your other customers we strongly advise to keep your address-quality up. (Otherwise Mailservers will Blacklist you)